ath9k: add workaround for AR9331 USB host controller hang with USB1.1 devices

 When wifi radio is reset, some bits in (undocumented) USB registers may
 flip. Patch adds setting them after radio reset (channel change,
 up/down).
 With this patch stability of USB1.1 full-speed devices, connected
 directly to AR9331 host port (no USB2.0 hub) is improved.